The present invention relates to a method and apparatus for drilling a secondary borehole from an existing borehole in a formation and more particularly to an improved milling and whipstock assembly for drilling a deviated borehole from an existing earth borehole or for side tracking through a cased borehole.
Whipstocks are known to have been used to drill a deviated borehole from an existing earth borehole. The whipstock has a ramp surface which is set in a predetermined position to guide the drill bit on the drill string in a deviated manner to drill into the side of the earth borehole. In operation, the whipstock is set in the existing earth borehole, the set position of the whipstock is surveyed, the whipstock is properly oriented for directing the drill string in the proper direction, and the drilling string is lowered into the well into engagement with the whipstock causing the whipstock to orient the drill string to drill a deviated borehole into the wall of the existing earth borehole.
Whipstocks are also known to be used for side tracking through a cased borehole. Previously drilled and cased boreholes, for one reason or another, may become nonproductive. When a wellbore becomes unusable, a new borehole may be drilled in the vicinity of the existing cased borehole or alternatively, a new borehole may be side tracked from or near the bottom of a serviceable portion of the cased borehole. Side tracking from a cased borehole may also be used for developing and drilling multiple production zones. Side tracking is often preferred because it avoids the additional drilling, casing and cementing of the borehole. The procedure for side tracking is generally accomplished by either milling out an entire section of casing followed by drilling through the side of the now exposed borehole or by milling through the side of the casing with one or more mills that are guided by a wedge or whipstock component.
Drilling a side tracked borehole through a cased borehole made of steel is difficult and often results in unsuccessful penetration of the casing and destruction of the whipstock. In addition, if the window is improperly cut, a severely deviated dog leg may result rendering the side tracking operation unusable. Patents which relate to methods and apparatus for sidetracking through a cased borehole include U.S. Pat. Nos. 4,266,621 and 4,397,355.
It is further preferred to perform the side tracking operation with one trip into the borehole. Such a one trip includes lowering the mills and whipstock into the borehole at the proper location, orienting the whipstock in the proper direction for cutting the cased borehole, setting and supporting the whipstock within the cased borehole, and milling the cased borehole to form a window in the casing.
U.S. Pat. No. 5,109,924 teaches a one trip window cutting operation for side tracking a cased borehole. A deflection wedge guide is positioned behind the pilot mill cutter and spaced from the end of the whipstock. The shaft of the mill cutter is retained against the deflection wedge guide such that the milling tool frontal cutting surface does not come into contact with the ramped base of the whipstock. In theory, the deflection wedge guide surface takes over the guidance of the window cutting tool without the angled ramp surface of the whipstock being destroyed. However, when a second and third milling tool attached to the same shaft as the window milling cutter and spaced, one from the other on the support shaft contacts the whipstock ramp surface, they mill away the deflection guide projection from the ramp surface. This inhibits or interferes with the leading pilot mill window cutter from side tracking at a proper angle with respect to the axis of the cased borehole and may cause the pilot window cutting mill to contact the ramp surface of the whipstock before the pilot window cutter mill clears the casing. The reamers or mills aligned behind the pilot window mill, having the same or larger diameter as the pilot window mill, prevents or at least inhibits the window pilot mill from easily exiting from the steel casing. This difficulty is due to the lack of clearance space and flexibility of the drill pipe assembly making up the one trip window cutting tool when each of the commonly supported reamer mills spaced along the shaft, sequentially contact the window in the steel casing. Hence, the side tracking apparatus tends to go straight rather than be properly angled through the steel casing.
U.S. Pat. No. 5,455,222 discloses a combination whipstock and staged side tracked mill. A pilot mill spaced from and located on the common shaft above a tapered cutting end is, at its largest diameter, between 50% and 75% of the final side track window diameter. A surface of a second stage cutter positioned on the same shaft above the pilot mill being, at its smallest diameter, about the diameter of the maximum diameter of the pilot mill, and being, at its largest diameter, at least 5% greater in diameter than the largest diameter of the pilot mill. A surface of a final stage cutter mill, also mounted on the same shalt, being at its largest diameter, about the final diameter dimension, and at the smallest cutting surface diameter, being a diameter of at least about 5% smaller than the final diameter dimension. The side tracking mill is designed to accomplish the milling operation in one trip. However, the mill tends to go straight and penetrates the ramp surface of the whipstock causing substantial damage to the whipstock occurs and preventing side tracking from occurring.
While it is intended that the prior art side tracking operations be performed in one trip, difficulties often arise when attempting to deviate the drill string from its original path to an off line side tracking path. Progressively larger in diameter reaming stages to enlarge the window in the steel casing inhibits the drill shaft from deviating or flexing sufficiently to direct the drill pipe in a proper direction resulting in damage to the whipstock and misdirected side track boreholes. In other words, the side tracking assembly tends to go straight rather than deviating through the steel casing.
The present invention overcomes the deficiencies of the prior art.